1. Field of the Invention
The present invention relates to an electromagnetic clutch and, more particularly, to an electromagnetic clutch used for connecting and disconnecting a compressor of an air conditioner in a vehicle.
2. Description of the Related Art
In a vehicle, an engine, which is a drive source, is used not only as a drive source for moving a vehicle but also as a drive source for various devices. There are many vehicles in which the compressor of an air conditioner for a vehicle is driven by an engine via a belt etc. In this case, the load applied to the engine can be reduced and fuel consumption can be improved by separating the compressor from the engine when it is not necessary to drive the air conditioner for a vehicle. It is also preferable, from the standpoint of life of the compressor to stop the compressor when not required. Because of such a need, an electromagnetic clutch is arranged between an engine and a compressor of an air conditioner for a vehicle for connecting and disconnecting the compressor.
However, when the compressor locks or if trouble occurs in the compressor without driver's knowledge, the electromagnetic clutch slips and produces heat and there may arise the possibility that another trouble, such as the breakage of a belt, occurs. Therefore, when an electromagnetic clutch is installed, measures against such problems must be taken.
Some conventional electromagnetic clutches use a temperature fuse or a lock sensor as a function for protecting a belt when the compressor locks. However, a temperature fuse poses a problem that it takes time to shut off power transmission and a lock sensor poses a problem that the cost is raised.
FIG. 10 shows one half of the sectional side elevation of a conventional example of a general electromagnetic clutch. In general, an electromagnetic clutch is substantially symmetrical in the vertical direction, therefore, only the upper half is shown. An electromagnetic clutch 100 in a conventional example shown in FIG. 10 comprises an electromagnetic coil 21 accommodated in a stator 22, a rotor 31 rotatively driven by a vehicle engine, which is not shown, an armature 14 that is attracted to the rotor 31 by the magnetic force of the electromagnetic coil 21, and a hub 6. The hub 6 has an outer plate 13 and an inner hub 12 and the outer plate 13 and the inner hub 12 are connected by welding a cylindrical rubber 11 thereto.
In the electromagnetic clutch 100 in the conventional example shown in FIG. 10, the outer plate 13 and the armature 14 are formed as separate parts and are attached by a plurality of rivets 15. Therefore, the armature 14 and the inner hub 12 rotate as one piece. The inner hub 12 is connected to the rotating shaft of the compressor of an air conditioner for a vehicle (not shown) and transmits the power of the engine to the compressor. The rotor 31 has a pulley 32 on its outer circumferential side. On one hand, a plurality of belts (not shown) are engaged with the pulley 32 and on the other hand, the belts are engaged with a pulley (not shown) on the engine side and, therefore, the drive force of the engine is transmitted to the pulley 32 and the rotor 31 via the belts. The operation of connecting and disconnecting the electromagnetic clutch 100 is performed by magnetizing the electromagnetic coil 21 to attract the armature 14 to the rotor 31 and by demagnetizing the electromagnetic coil 21 to separate the armature 14 from the rotor 31.
In the conventional example shown in FIG. 10, the outer plate 13 and the armature 14 are formed as separate parts and are attached with the rivets 15. In this structure, when the compressor locks, the heat produced at the frictional surface between the rotor 31 and the armature 14 is unlikely to be transmitted to the cylindrical rubber 11 and it takes time for the cylindrical rubber 11 to melt and shut off power and, therefore, there is the possibility that the belt is damaged.
For example, there is a prior art similar to the above-mentioned prior art, in which a cylindrical rubber damper is provided (for example, refer to Japanese Unexamined Utility Model 53-39822). In this art, as there is a gap between an armature and an outer plate, heat is unlikely to be transmitted and, therefore, it is difficult to shut off power transmission. Another art is proposed (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 2000-161389A). In this cited art, an elastic member is melted to shut off power transmission. However, there arises a problem that the damping effect is slight and the performance of absorbing torque variations is poor although the member is readily melted to shut off power transmission.
[Patent document] Japanese Unexamined Utility Model 53-39822
[Patent document] Japanese Unexamined Patent Publication (Kokai) No. 2000-161389A